1. Field of the Invention
The invention relates to the formation of products from powder slurries.
2. Description of Related Art
Ceramic components, ranging from silicon nitride turbocharger rotors, used in high-performance automobiles, to translucent aluminum oxide tubes, used in high efficiency yellow sodium lamps, are formed by molding a powder into the desired engineering shape. The powder compact is then densified to its final form by a high temperature heat treatment. Because advanced ceramic powders such as silicon nitride and alumina lack the plastic properties of traditional clay-based ceramics, conventional shape forming is carried out by either the pressure consolidation of a dry powder or by the pressure forming of a powder containing a large fraction of a polymer that imparts plasticity. As received, ceramic powders contain agglomerates, inorganic and organic inclusions, and other inhomogeneities that degrade both the electrical and mechanical properties of sintered, dense bodies. As a result, both of these commercial shaping methods suffer from inclusions, present in the powder and retained during shape forming and densification. These inclusions concentrate any applied stress to severely degrade the component""s strength. Agglomeration via spray drying prior to dry pressing incorporates more contaminants from the drying air, and crack-like void spaces are generated when the spray-dried agglomerates do not fully deform. In addition, the large quantity of polymer required for plastic forming (on the order of 40% by volume) must be removed very slowly to avoid defect formation.
It is known that the reliability of ceramic components can be improved by processing the powder as a colloidal suspension [F. F. Lange, xe2x80x9cPowder processing science and technology for increased reliability,xe2x80x9d J. Am. Ceram. Soc. 72, 3 (1989)]. When the ceramic particles are dispersed in a liquid, the slurry can be passed through a filter to remove all inclusions greater than the size defined by the filter. Reducing the inclusion size will increase the average strength and component reliability. Techniques currently employed to form engineering shapes from a slurry can be categorized as either consolidation or direct shaping methods.
Consolidation methods start with a slurry containing a low volume fraction of powder that is concentrated by either evaporation or pressure filtration. Examples include tape casting (evaporation) [R. E. Mistler, D. J. Shanefield, R. B. Runk, xe2x80x9cTape casting of ceramics,xe2x80x9d in Ceramic Processing Before Firing, G. Y Onoda, L L Hench Eds., (Wiley-Interscience, New York, 1978) pp. 411-448], slip casting (for the production of ceramic green parts, in which hardening is achieved, as is well known, by water removal with capillary pressure via a porous mold) [J. S. Reed, in Principles of Ceramic Processing (Wiley-Interscience, New York, ed. 2, 1995) pp. 493-503], and pressure filtration (external overpressure) [F. F. Lange, K. T. Miller, xe2x80x9cPressure filtration: Consolidation kinetics and mechanics,xe2x80x9d Am. Ceram. Soc. Bull. 66, 1498 (1987)]. Because the initial volume fraction of powder is  less than 0.40, these dispersed slurries can first be passed through a filter to remove strength degrading inclusions. Consolidation methods also have the capability to produce bodies with the highest relative density. However, because the liquid removed during consolidation must flow through the body as it consolidates, these methods generally require long periods within the mold. Additionally, tape casting and slip casting, are typically limited to thin (or thin walled) bodies.
Direct shaping methods start with a slurry containing a high volume fraction of powder ( greater than 0.50) that can still be either poured or injected into a mold. Unlike the consolidation methods, the volume fraction of powder does not change during molding. Highly repulsive interparticle potentials are needed to formulate flowable slurries containing a high volume fraction of powder. Within the mold, the slurry must be converted to an elastic body so the component can retain its shape upon removal from the mold. Direct shaping methods include injection molding, gel casting, direct coagulation casting and vibra-forming [J. A. Mangels, xe2x80x9cInjection molding ceramics,xe2x80x9d Ceram. Eng. Sci. 3, 529 (1982); A. C. Young, O. O. Omatete, M. A. Janney, P. A. Menchhofer, xe2x80x9cGelcasting of alumina,xe2x80x9d J. Am. Ceram. Soc. 74, 612 (1991); T. J. Graule, F. H. Baader, L. J. Gauckler, xe2x80x9cShaping of ceramic green compacts direct from suspensions by enzyme catalyzed reactions,xe2x80x9d cfi/Ber. DKG 71, 317 (1994); and G. V. Franks, B. V. Velamakanni, F. F. Lange, xe2x80x9cVibraForming and in-situ flocculation of consolidated, coagulated alumina slurries,xe2x80x9d J. Am. Ceram. Soc. 78, 1324 (1995)]. In the case of injection molding and gel casting, the slurry""s liquid phase solidifies, respectively, by freezing or polymerization. For direct coagulation casting and vibra-forming, the particle network within the slurry is solidified by changing the pH of the slurry to the isoelectric point (the pH where the net surface charge on the particle is zero) via a temperature induced chemical reaction. Because slurries used for all direct shaping methods must contain the highest volume fraction of powder possible, they are too viscous to remove strength degrading inclusions by filtration. Although injection molding only requires very short periods within the mold, very long periods are needed to remove the polymer without causing cracking, blistering, etc. The periods required to convert the molded slurry into an elastic body is too long (10 minutes to several hours) to utilize either gel casting, direct coagulation casting or vibra-forming as rapid forming methods.
After a component is shaped by any one of these methods, the liquid within the powder compact must be removed before densification at high temperature. Shrinkage typically occurs during evaporative drying because the powder can further consolidate, driven by capillary (Laplace) pressure. Surface tensile stresses associated with shrinkage may arise if the exterior portion of the body, where evaporation initiates, is constrained by the interior. The magnitude of the tensile stress, which can induce cracking, depends on a number of factors including the initial relative density achieved during shape forming and the rate of drying [G. W. Scherer, xe2x80x9cTheory of drying,xe2x80x9d J. Am. Ceram. Soc. 73, 3 (1990)]. Bodies produced by direct shaping methods are more prone to cracking because they exhibit greater shrinkage relative to the higher density, consolidated bodies.
Pujari et al., have shown that the average strength and reliability of tensile specimens can be greatly improved by filtering inclusions from slurries prior to compact formation by pressure filtration [V. K. Pujari et al., xe2x80x9cReliable ceramics for advanced heat engines,xe2x80x9d Am. Ceram. Soc. Bull. 74, 86 (April 1995)]. Unfortunately, most of the colloidal forming methods described above have only found niche industrial applications. For example, tape casting is one viable technique for fabrication of multilayer electronic packages. In general, the long forming periods, large fractions of polymer that must be removed prior to densification, and/or the inability to remove inclusions prior to shaping are the limiting factors for economical, industrial practice.
The present invention provides a new method to form ceramic components from colloidal suspensions of ceramic powders, in which isostatic, pressure is applied to a colloidal suspension, and which we call colloidal isopressing. The method starts with a slurry that can be filtered to remove strength degrading inclusions. After an initial, low pressure consolidation, which can use pressure filtration to create a fluid-like consolidated body, the shape forming method requires only a short isopressure period within a flexible mold. Following shaping, the saturated body can be rapidly dried without shrinkage, or in accordance with an embodiment of the invention, heated directly to the densification temperature. Post-processing machining is minimized or eliminated because the shape and contours imparted by the mold are retained during drying and densification. The invention is exemplified with water as the fluid, preferred for environmental and health reasons, but the process can be conducted with other solvents and liquids. The invention is not limited to a specific fluid; in some cases, e.g., where the powder reacts or dissolves in water, organic fluids can be used.
Two phenomena, discovered in previous studies, enable this new forming technology. The first is a method to produce a repulsive interparticle pair potential that persists after pressure consolidation. The second is the discovery that a critical consolidation pressure exists that separates plastic and brittle behavior. The present invention applies a new forming method using these phenomena, illustrated with alumina powder, silicon nitride powder, zirconia powder, and silicon nitride aqueous slurries. Alternative ceramic powders can be used, including silicon carbide, aluminum nitride, titania, barium titanate, zinc oxide, and lead-zirconate-titanate. Powder or particles other than ceramics can be used, for example , and organic polymers, such as poly(ether ether ketone) and Teflon, where the short-range repulsive potential is developed by steric methods. In accordance with this invention, the particles are attracted to one another in a slurry formulation by the pervasive van der Waals potential, but they are prevented from touching with a short range repulsive potential. The van der Waals potential always causes particles of the same material to be attractive when the surrounding fluid has a different dielectric constant. By itself, the van der Waals potential produces a network of particles in elastic contact. Due to friction, touching particles are difficult to rearrange during consolidation and therefore do not produce the highest relative density [J. C. Chang, F. F. Lange, D. S. Pearson, J. P. Pollinger, xe2x80x9cPressure sensitivity for particle packing of aqueous Al2O3 slurries vs. interparticle potential,xe2x80x9d J. Am. Ceram. Soc. 77, 1357 (1994)]. To keep the particles apart, low density matter that does not significantly contribute to the van der Waals potential shrouds the particles and causes an increase in the free energy when the shrouds of approaching particles interact. The interpenetrating shrouds keep the particles apart and, in effect, shield either a portion or all of the attractive van der Waals potential. When the electrostatic double layer method (approximated by the DLVO theory) is used to produce a repulsive potential, counterions comprise the shroud [R. G. Hom, xe2x80x9cSurface forces and their action in ceramic materials,xe2x80x9d J. Am. Ceram. Soc. 73, 1117 (1990)]. When the steric approach is used, the shroud is composed of molecules, e.g. linear molecules bonded to the surface and extending into the surrounding fluid.
Colloidal isopressing has attributes of both the consolidation and direct shaping methods. It is known that particles held apart by a short-range repulsive potential can be pushed into contact during consolidation when the applied pressure is greater than a critical value [G. V. Franks, F. F. Lange, xe2x80x9cPlastic-to-brittle transition of saturated, alumina powder compacts,xe2x80x9d J. Am. Ceram. Soc. 79, 3161 (1996)]. The critical transition pressure between plastic and brittle behavior was determined by filter pressing dilute slurries at varying pressures and mechanically testing the formed compacts in uniaxial compression. Bodies were either plastic, i.e., would flow at a yield stress, or crack. The consolidation pressure that separates the plastic behavior from the cracking (brittle) behavior is the critical transition pressure. When the slurry is consolidated below the critical pressure, the consolidated body can be made to flow at a stress that is governed by the interparticle pair potential formulated in the slurry state. At pressures above this critical value, the consolidated body exhibits elastic behavior. We use this plastic-to-elastic transition by first consolidating a body below the critical pressure, injecting the fluid-like body into a flexible mold, e.g. of rubber, then isopressing the filled mold above the critical pressure. The high isostatic pressure acts to convert the fluid-like material to an elastic body that can be removed from the rubber mold without shape distortion. Above the critical consolidation pressure, the particles are forced into contact and pack to their highest relative density. Therefore, the liquid that remains can be rapidly removed by evaporation without either shrinkage or cracking.
Implementing colloidal isopressing does not require the development of new capital equipment: filter (de-watering) presses are used in the clay industry to consolidate clay slurries, and isostatic presses are used to produce millions of spark plug insulators daily.
These and other features, aspects, and advantages of the present invention will become better understood with regard to the following detailed description, appended claims, and accompanying drawings.